He is right, much of Cozumel has a constant current (we drift dive and the boat follows us scuba divers) and the natural bits are what we call "iron shore" (no sand). If you want to snorkel, one of the nicest ways is to go to Chankunaab park. You can take a taxi from the docks. Be aware that there is a charge to get in (a bit over $20 per person) but it is nice white sand, beach chairs and friendly staff. They even have a package for entrance and snorkeling gear so you don't have to take your own.